doc: raise doxygen lookup cache

The lookup cache size is exhausted again, bumping it from:

   131072 entries - 2^(16+1)
to 262144 entries - 2^(16+2)

See previous bump for details:
    432a0cb https://gerrit.wikimedia.org/r/69709

Change-Id: I9a15cce6f77745c4e913a562e8f1f43a4988c83d
Merge "doc: raise doxygen lookup cache"